This invention relates to a tensioning and drawing-back device provided for keeping under tension and if necessary drawing back at least one warp yarn led from a bobbin to a weaving machine.
More especially this invention relates to a device which is provided for tensioning and drawing back several warp yarns coming from respective bobbins on a creel to a weaving machine.
These types of devices are generally known and have among others an important application with jacquard weaving machines with different yarn consumption for each individual warp yarn.
Known tensioning and drawing-back devices are for example provided on a creel disposed behind the weaving machine for tensioning and drawing back the warp yarns which during the weaving process are led from the bobbins on this creel to the weaving machine. A creel supports a great number of bobbins or packages. The various warp yarns which are required for weaving a fabric are led from respective bobbins via a number of guiding grids to the weaving machine. In order to prevent these warp yarns from entangling together they must continuously be held under tension. Slack warp yarns in the weaving area must also be absolutely avoided because they adversely affect the fabric quality. They can for example result in an irregular pile formation. In particular warp yarns which on weaving occupy different successive positions in the shed must be capable of being drawn back out of the weaving area in order to keep them under tension.
In a known embodiment each warp yarn unwound from a bobbin is first passed around a guiding spindle disposed behind this bobbin and subsequently brought over the bobbin and a guiding spindle disposed in front of the bobbin. A first and a second drop wire are suspended from the warp yarn, respectively between the bobbin and the rear guiding spindle and between the bobbin and the front guiding spindle, so that these drop wires can press the interjacent piece of warp yarn, which extends above the bobbin, onto the bobbin and in so doing can form a type of band brake on the winding surface of this bobbin. This occurs if the warp yarn is slack.
If the warp yarn tightens during the weaving process, whereby the drop wires are raised, the warp yarn comes into a position whereby it is no longer in contact with the winding surface of the bobbin, so that the braking effect is terminated. Through the further unwinding of the warp yarn the tension can fall away, so that the warp yarn again comes to hang slack. Through the downward movement of the front drop wire the warp yarn is again tensioned and if necessary drawn back out of the weaving area.
A creel provided with such a tensioning and drawing-back device has the disadvantage that the drawing-back effect achieved by the drop wires is dependent on the location of the bobbin in the creel. A drop wire with a bobbin placed at the rear in the creel will have much less effect than one and the same drop wire with a bobbin placed at the front in the creel. The number of points of friction between the drop wire and the weaving area is indeed much greater for the warp yarn which is unwound from the rearmost bobbin.
Another significant disadvantage of this known device is that the replacement of a bobbin is a rather difficult and time-consuming work.
In the European patent application EP 0 742 297 a tensioning and drawing-back device has been described which can be disposed independently of the bobbins and detached from the creel. Each warp yarn is led through a feed-through eye of a strip. This strip is provided capable of sliding up and down in a holder and exerts a tensile force on the warp yarn between two guiding grids. The tensile force is implemented by the own weight of the strip or by means of a retracting spring connected to the strip and a fixed point of the device. Through this tensile force the warp yarn is tensioned and if necessary drawn back out of the weaving area.
Such a device can be disposed at the front in the creel. The replacement of an empty bobbin is easier and less time-consuming with this device than with the above described device. Furthermore in this manner a drawing-back force is achieved which is independent of the location of the bobbin in the creel.
In order to prevent the warp yarns from being pulled out of both the weaving area and from the bobbin to the strip, through which the warp yarn would not be drawn back out of the weaving area in an effective manner and fabrics of less good quality would be woven, with this device, along the side of the bobbin, between the first guiding grid and the bobbin, a leaf spring brake is provided in order to retain the warp yarn. Since tensioning and drawing-back devices are generally implemented for rather large numbers of warp yarns this makes the device much more complex and expensive.
With the embodiments with retracting spring the disadvantage furthermore also exists that the warp yarn tension with the passage of time is subject to too much change through extension of the retracting springs.